In general, in a mobile communication system, a random access procedure is used for an initial connection. In a mobile communication system employing an LTE (Long Term Evolution) scheme, a channel for the random access procedure is called PRACH (Physical Random Access Channel). Furthermore, in the LTE scheme, the random access procedure is also used for a handover, data communication resumption and the like, in addition to the initial connection.
The LTE mobile communication system is configured to assign one or a plurality of Zadoff-Chu sequences to each cell as “RACH root sequence” of the PRACH so as to ensure 64 RACH preambles per one cell.
Here, the Zadoff-Chu sequence has characteristics that the amplitude is constant in a temporal domain and a frequency domain and self-correlation is 0, and sequences obtained by performing a cyclic shift with respect to the Zadoff-Chu sequences are orthogonal to one another. There are 838 types of the Zadoff-Chu sequences.
Specifically, as illustrated in FIG. 9, a cyclic shift is performed with respect to one or a plurality of Zadoff-Chu sequences by a predetermined cyclic shift amount (12.5 μsec in the example of FIG. 9), so that it is possible to ensure the 64 RACH preambles per one cell. Here, a value such as 800 μsec or 12.5 μsec is for illustrative purposes only.
In addition, the number of RACH preambles generable from one Zadoff-Chu sequence depends on a cell configuration (for example, a cell radius, a propagation delay state, the presence or absence of high speed movement compatibility).
Here, when a propagation delay between a radio base station eNB and a mobile station UE is larger than the above-mentioned cyclic shift amount, it is not possible for the radio base station eNB to recognize whether propagation is delayed by a cyclic shift or an actual propagation delay.
Specifically, as illustrated in FIG. 10(a), when a propagation delay is larger than the above-mentioned cyclic shift amount Ncs, it is not possible for the radio base station eNB to distinguish an RACH preamble #1 (an RACH preamble #1 from a mobile station UE positioned at the center of a cell), which has a cyclic shift amount of “Ncs”, from an RACH preamble #2 (an RACH preamble #2 from a mobile station UE positioned at the end of a cell), which has a cyclic shift amount of “0”, in a reception window #102.
In such a case, at the time of a cell design work, as illustrated in FIG. 10(b), in order to allow the cyclic shift amount Ncs to increase and ensure 64 RACH preambles per one cell, it is necessary to increase the number of Zadoff-Chu sequences to be assigned to the cell.
As described above, in the LTE scheme, it is necessary to perform design, that is, perform planning, of an RACH root sequence of PRACH to be assigned to each cell. Furthermore, in the design of the RACH root sequence, it is necessary to consider a cell configuration (for example, a maximum transmission delay in a cell).
However, in general, in the step of the design of the RACH root sequence, it is difficult to accurately estimate the above-mentioned maximum transmission delay and the like in the cell. Furthermore, after a mobile communication system starts to operate, the cell configuration (for example, the maximum transmission delay in the cell) may be changed. Even in this case, it is difficult to accurately estimate the above-mentioned maximum transmission delay and the like in the cell.
In such a case, conventionally, since it is necessary for a network operator to perform field measurement and the like and perform a cell design work (specifically, an assignment work of an RACH preamble) again, great effort is required, resulting in an inefficiency problem.